Effect of Coal Chemistry on Carbonization Behaviour and Association with Coke Characteristics

Thermoplastic behaviour of coal plays a critical role on developing high coke quality attributes including strength which are critical for efficient blast furnace operation. Effect of coal plasticity on the evolution of coke microstructure is complex and particularly limited in relation to role of coal mineral chemistry. This study reports an experimental investigation relating to coal phase transformations during controlled carbonization as a function of coal ash and mineral composition using a range of analytical tools including FTIR, GC/MS, XRD, SEM and Optical microscopy. The chemical nature and magnitude of various carbonization products were related to the microstructure and the mechanical strength their cokes. The study identified the significance of coal mineral chemistry on plasticity as well as coke microstructure and mechanical strength. A number of microstructure parameters were combined to develop a combination microstructural parameter, and related to coke tensile strength as well as plastic range of parent coals. Under the tested conditions, up to 5 wt. % of kaolinite addition in coal is found to improve the fluidity, which could be associated with the increase of CHAr/CHAl ratio as well as decrease in CH3/CH2 ratio of the residual char. The proportion of C=O/C=C bonds affected the plastic range. The study further showed that elevated kaolinite level in coal suppresses the evolution of low molecular compounds in tar phase and decrease the fluidity. Kaolinite addition in coal showed a significant effect on altering tar composition particularly phenol and benzene content especially at lower temperatures. Increasing presence of kaolinite levels in coal increased coke tensile strength but did not show a similar level of improvement in case of conventional coke tumbling strength. The effect of kaolinite presence on coal pyrolysis behaviour was also influenced by particle size such that finer particle size promoted fluidity while coarse particles improve the release of hydroaromatic methylene compounds. Coarser kaolinite leads to improve coke microstructure as well as mechanical strength. The study has implications for coal blending practice for improving coke quality such that future studies may focus in clarifying the role of other coal minerals and their associations on their thermoplastic behaviour